Motion sensors, which include gyroscopes and their components (e.g., angular rate sensors and accelerometers), are widely used in VCR cameras and aerospace and automotive safety control systems and navigational systems. Examples of automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint (SIR) systems such as air bags and seat belt lock-up systems, and crash sensing systems. Automotive yaw rate sensors sense rotation of an automobile about its vertical axis, while accelerometers are used to sense acceleration and deceleration of an automobile.
In the past, electromechanical and electronic motion sensors have been widely used in the automotive industry to detect an automobile's deceleration. More recently, sensors that employ an electrically-conductive, micromachined plated metal or silicon sensing element have been developed which can be integrated with bipolar/CMOS/BiCMOS circuits on a wafer. An example of a plated metal surface micromachine is disclosed in U.S. Pat. No. 5,450,751 to Putty et al. assigned to the assignee of this invention. The disclosed micromachine is formed by a metal plating technique in cooperation with a mold that defines the shape of the micromachine on the surface of a wafer. Putty et al. further disclose a novel configuration for the micromachine, which includes a resonating metal ring and spring system. A variation of the sensor disclosed by Putty et al. is described in U.S. Pat. No. 5,547,093 to Sparks, in which an electrically-conductive, micromachined silicon sensing element is disclosed. Sparks micromachined sensing element is formed by etching a "sensing" chip formed of a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer.
The sensors of both Putty et al. and Sparks employ an electrode pattern composed of a number of individual electrodes along the perimeter of the ring. A capping wafer can be used to enclose the ring within an evacuated cavity defined by and between the sensing and capping wafers. Conductive runners on the sensing chip enable the electrodes to be electrically interconnected with appropriate signal conditioning circuitry and to provide a biasing voltage to the ring. In operation, some of the electrodes serve as "drive" electrodes that drive the ring to resonate when the electrodes are appropriately energized. Other electrodes serve as "balance" electrodes that when energized serve to balance the resonant peaks of the flexural movement of the ring by changing the electromechanical stiffness of the ring and springs. Still other electrodes are "sensing" electrodes that capacitively sense the proximity of the ring relative to the sensing electrodes. With the above construction, a sensor is able to detect movement of the ring toward and away from the sensing electrodes, which occurs in response to the angular velocity of the ring about its axis of rotation due to the effects of the Coriolis force. Consequently, when properly installed, the sensor is able to sense rotation rate about any chosen axis of an automobile.
Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in automotive applications. However, the intricate sensing element required for such sensors must be precisely formed in order to ensure the proper operation of the sensor. For example, a sufficient gap must exist between the electrodes and the sensing element ring to prevent shorting, yet sufficiently close to maximize the capacitive output signal of the sensor. In addition to these operational considerations, there is a continuing emphasis for motion sensors that are lower in cost, yet exhibit high reliability and performance capability. The cost of a sensor is strongly impacted by its process yield, which in turn is a function of the parameter sensitivities of the sensor with temperature. Temperature sensitivities are present due to the narrow gap required between the ring and its drive, balance and sensing electrodes, the effect of which is compounded by the large length ratios between the ring and electrodes. The natural mode frequency of the ring is also affected by temperature, which can impact the scale factor response of the ring at resonance.
Therefore, it would be highly desirable if further advancements could be made toward reducing the sensitivity of the above-described motion sensors to temperature variations in their operating environment.